The sensitivity of a capacitor microphone in a high frequency circuit, referred to for brevity as an HF capacitor microphone, substantially depends on a high frequency voltage applied to a capacitor transducer of the capacitor microphone, the HF voltage in turn depending on the efficiency of the HF circuit.
If the capacitor microphone is manufactured in mass production it usually happens that there is scatter in the efficiency of the HF circuit. That can result in failure of the capacitor microphone in the circuitry and microphone test, particularly if the sensitivity differs excessively from the intended target value.
With the influence of climate on the losses in the components of the HF circuit the efficiency of the HF circuit and therewith also microphone sensitivity become climate-dependent.
It can further happen that an externally employed phantom voltage for the capacitor microphone does not afford sufficient power so that the HF voltage can drop and therewith also microphone sensitivity.
FIG. 1 shows a diagrammatic view of a capacitor microphone in accordance with the state of the art. The capacitor microphone has a symmetrical capacitor transducer KW with a diaphragm M. The capacitor microphone further has an HF bridge HFB which is formed by the capacitor transducer and the primary side of an HF coil HFS with an adjusting core. On the secondary side the HF coil HFS is coupled to an HF oscillator HFO and a synchronous demodulator SD, with two diodes D1, D2 and two capacitors and two resistors. The output of the synchronous demodulator SD is coupled to the low frequency output NFA. The capacitor microphone further has an HF transformer HFT for the HF bridge signal. In that respect the primary side of the HF transformer HFT is coupled to the primary side of the HF coil HFS and the diaphragm and the secondary side of the HF transformer HFT is coupled to the secondary side of the HF coil HFS and the low frequency output NFA. The secondary side of the HF transformer HFT is coupled to the low frequency output NFA. A capacitor C is provided at the low frequency output NFA.